As a method of preserving a cell, a method of freezing and preserving a cell, such as a low-temperature preserving method or an ultralow-temperature preserving method, is generally performed. As a freezing method, both a slow freezing method and a vitrification freezing method are well known. The slow freezing method cools a cell at a slow cooling rate for a long time, and inhibits the formation of large ice crystals in the cell. Thereby, damage (freezing damage) to the cell due to freezing is assumed to be avoidable. However, the slow freezing method requires a long time until freezing is completed, and requires the use of an expensive freezer or the like having a special function to control the cooling rate. As an alternative method to overcome the disadvantages in the slow freezing method, a vitrification freezing method has been widely used in recent years. The vitrification freezing method is a method of quickly freezing a cell processed with a vitrification solution using liquid nitrogen and thereby freezing the cell without forming ice crystals inside and outside the cell. This uses a phenomenon called vitrification, that is, a phenomenon in which, when a solution is rapidly cooled, the solution quickly passes through an original freezing point to cause supercooling, and movement of water molecules comes to a stop without forming ice crystals. This method is assumed to be superior to the slow freezing method in that damage to the cell caused by formation of ice crystals does not occur, in that a time required for processing is short, and in that a special equipment is not required. Multiple specific vitrification freezing methods for the cell have been developed. As one example, there is a method of dipping the cell into a freezing-resistant preserving solution and rapidly freezing in liquid nitrogen or in an ultralow freezer.
In cryopreservation of an ovum, cryopreservation using an exclusive instrument is performed. For example, as an exclusive instrument that is currently used in a cryopreservation method of the ovum, there is a Cryotop (registered trademark). Cryotop is an exclusive instrument (Cryotop) in which an extremely thin strip-like sheet is attached to a tip of a handle. Generally, the cryopreservation method using Cryotop is called a Cryotop method. In the Cryotop method, for example, a freezing fluid including a cryoprotectant and an ovum are placed on the sheet of the Cryotop, and the sheet on which the ovum is placed is dipped into liquid nitrogen and is vitrified and frozen. Then, the sheet is housed and protected in a sheath-like container to protect a sheet portion, and is kept in the liquid nitrogen or the freezer.
The inventors of the present invention have recently developed a hollow fiber cryopreservation method which is a cell cryopreservation method (Non-Patent Literature 1). This is a method of introducing a cell such as an egg cell into a hollow fiber made of a cellulose acetate membrane and cryopreserving the cell in a state in which the egg cell is included in the hollow fiber. The hollow fiber cryopreservation method is a highly reliable method capable of improving cell viability, and dissemination thereof is eagerly anticipated.